This invention relates to the production of control signals for operating a visual display mechanism of the scanning type, such as a standard television receiver, and, more particularly, to a method and apparatus for composing under the control of a microprocessor each frame of a display at the same time the display surface area is being scanned to produce the same.
Microprocessor controlled units are now available for producing the scanning signals required to form a display on a standard television receiver. While such units typically are used to produce displays for video games and the like, proposals have been made to provide a much more powerful control unit which will, in effect, turn a television receiver into a visual output device of a computer. That is, it has been proposed that the control unit not only have the processing power to compose those displays incident to video games, but also have the processing power to perform many calculations and functions which may be desired in the home, and to produce on the television receiver whatever displays may be incident thereto.
It will be recognized that a unit for producing scanning control signals for a television receiver must be quite versatile in order to enable such usage of a standard television receiver. U.S. Patent Application Ser. No. 755,749, filed Dec. 30, 1976 and assigned to the same assignee as this application, describes and claims a control method and apparatus which has such versatility. The invention of such application comprises composing each of the frame displays during the very scan which produces the same. It accomplishes this by storing information defining a plurality of different display segments which it may be desired be included in any particular display, and then selectively extracting such information when it is required to produce the particular display segments which are to appear in any one television display. This is in contrast to prior microprocessor controlled arrangements in which each frame display is, in effect, duplicated in memory prior to the time that it is actually displayed.
Standard television receivers typically produce thirty display frames per second, each one of which requires 525 scan lines. Many of such receivers are capable of resolving each scan line into 320 individual display points which can be separately defined. Therefore, the time between the display of adjacent display points is in the order of 4.5.times.10.sup.-6 seconds, very little time in which to both make decisions on the difference between adjacent display points and to actually make the control changes necessary to reflect such differences on the screen. One major problem in taking full advantage of the resolution and versatility available in standard television receivers is the difficulty of inexpensively making such decisions and changes.